How to make a cable to record cockpit intercom?

Question

I would like to record the cockpit intercom with my iPhone and a recording app. I do not know much about impedance matching (other than what it is).

I would like to make a cable that plugs into the audio out jack to my headset and has an output to a 3.5mm jack as well as a pass-through wire so I can still use a headset.

I would like to make one of these, but I'm not sure what sort of resistors they are using inside of it.

I can tell you that the impedance of the headset is 600ohms, which means that the signal going to them has a very high voltage that I need to step down before I can record it with an iPhone.

I'm sorry for the lack of information or experience, but I do not want to blow $40 on a cable that should cost $3.

How good (as in quality level) does the recording need to be? The signal can be split between two outputs (headphones + recording device) without any circuitry at all - just splicing wires. In fact, I think they sell these cheap cables at dollar stores for splitting an MP3 player output for two sets of headphones. However, the power of the signal will be split as well, with the amount of splitting proportional to the impedance difference between the headset and the recording device. — Kurt E. Clothier, May 02 '13 at 06:09
Aren't planes subject to periodic inspection and will an inspector like the fact that you have been soldering wires yourself, deviating from the design/documentation? — jippie, May 02 '13 at 06:57
@jippie I raised a similar concern on one of his previous questions :-) His response: He'll be piloting the flyer. — Anindo Ghosh, May 02 '13 at 08:17
If you look at the link I provided you will see that I am not going to be messing with the airplane's built in electronics. — Sponge Bob, May 02 '13 at 14:04

score 3 · Accepted Answer · edited Apr 13 '17 at 12:33

Please refer to my answer to your previous question on the subject:

If your recording device is high impedance relative to 600 Ohms it has no significant impact on impedance matching for the existing output source or your headphones, the impedance stays matched. This is easily achieved by using a voltage follower as buffer on the recording device, if its impedance is not high enough (anything over 6 kOhms will ensure less than 10% attenuation). Most recording devices will already cover this.

Having said that, all you absolutely need to make a splitter for splicing in the recorder is two 3.5 mm connectors (one male and one female), and one connector for the recording device. The recording device connector is spliced in, in parallel to the wires to your splitter's headphone socket.

As previously suggested, you would need to measure the actual signal voltage carried on the headphone cable - "a very high voltage" is rather unlikely in a headphone connection. Once that voltage is determined, you can add a suitable voltage divider to the signal line going to the recording device, to bring the signal down to a required voltage level, if needed.

Adding a resistor, say 6.8 kOhms or higher, in series with the signal line on the recording device connector's wire pair will add a safety element, in that even if the recording device becomes a short circuit for some reason, there will not be perceptible attenuation on the headset, and the output device will not be harmed. This series resistor can well be part of the voltage divider suggested above, if the divider is required at all.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

This will of course reduce the signal strength reaching the recording device, but whether that attenuation leaves you with a usable signal is best determined by experimenting.

The above example schematic attenuates a 24 Volt peak to peak signal to 2 Volts peak to peak for the recorder - if those are what the output device generates and what the recording device needs, you're all set.

So in short, you really do not need a special purpose splitter, unless you want the pre-packaged goodness and the convenience of a coiled wire, all done for you.

How to make a cable to record cockpit intercom?

1 Answers1